A hydrogen generation assembly, or hydrogen-producing fuel processing assembly, is an assembly that converts one or more feedstocks into a product stream containing hydrogen gas as a majority component. The produced hydrogen gas may be used in a variety of applications. One such application is energy production, such as in electrochemical fuel cells. An electrochemical fuel cell is a device that converts a fuel and an oxidant to electricity, a reaction product, and heat. For example, fuel cells may convert hydrogen and oxygen into water and electricity. In such fuel cells, the hydrogen is the fuel, the oxygen is the oxidant, and the water is a reaction product. Fuel cells are typically coupled together to form a fuel cell stack.
A hydrogen-producing fuel cell system is a hydrogen-producing fuel processing system that also includes a fuel cell stack that is adapted to receive hydrogen gas produced by the fuel processing assembly and to generate an electric current therefrom. The hydrogen-producing fuel cell system includes a hydrogen-producing region in which hydrogen gas is produced as a majority reaction product from one or more feedstocks. The composition, flow rate, and properties of the feedstock(s) delivered to the hydrogen-producing region may affect the performance of the hydrogen generation assembly. This, in turn, may affect the performance of the fuel cell stack, the hydrogen-producing fuel cell system, and/or its ability to satisfy an applied load thereto. Accordingly, hydrogen-producing fuel processing assemblies and hydrogen-producing fuel cell systems will typically include various controls for regulating the flow of feedstock to the hydrogen-producing region.
To efficiently produce hydrogen gas, the feedstock(s) for a hydrogen-producing fuel cell system should be delivered under the desired operating conditions, including temperatures and pressures in a predetermined range, to a hydrogen-producing region of the fuel processing assembly. The product hydrogen stream from the hydrogen-producing region may be purified, if needed, and thereafter used as a fuel stream for a fuel cell stack, which produces an electric current from the product hydrogen stream and an oxidant, such as air. This electric current, or power output, from the fuel cell stack may be utilized to satisfy the energy demands of an energy-consuming device.
A consideration with any hydrogen-producing fuel cell system is the time it takes to begin generating a desired electric current from hydrogen gas produced by the fuel cell system after there is a need to begin doing so. In some applications, it may be acceptable to have a period of time in which there is a demand, or desire, to have the fuel cell system produce a power output to satisfy an applied load, but in which the system is not able to produce the desired power output from hydrogen gas produced by the fuel cell system's fuel processing assembly. In other applications, it is not desirable or acceptable to have such a period where the applied load from an energy-consuming device cannot be satisfied by electric current produced from the hydrogen gas being concurrently generated by the hydrogen generation assembly. As an illustrative example, some fuel cell systems are utilized to provide backup, or supplemental power, to an electrical grid or other primary power source. When the primary power source is not able to satisfy the applied load thereto, it is often desirable for the backup and/or supplemental fuel cell system to be able to provide essentially instantaneous power so that the supply of power to the energy-consuming devices is not interrupted, or not noticeably interrupted.
Fuel cells typically can begin generating an electric current within a very short amount of time after hydrogen gas or another suitable fuel and an oxidant, such as air, is delivered thereto. For example, a fuel cell stack may be adapted to produce an electric current within less than a second after the flows of hydrogen gas and air (or other oxidant stream) are delivered to the fuel cells in the fuel cell stack. Inclusive of the time required to initiate the delivery of these streams from a source containing the hydrogen gas and air, the time required to produce the electric current should still be relatively short, such as less than a minute. However, hydrogen-producing fuel cell systems that require the hydrogen gas to first be produced, and perhaps purified, prior to being utilized to generate the desired power output take longer to generate this power output. When the fuel processing assembly is already at a suitable hydrogen-producing temperature, the fuel cell system may be able to produce the desired power output from hydrogen gas generated by the fuel processing assembly within a few minutes, or less. However, when the hydrogen-producing fuel processor of the fuel cell system's fuel processing assembly is not already at or near a desired hydrogen-producing temperature, the required time will be much longer.
Conventionally, several different approaches have been taken to provide hydrogen-producing fuel cell systems that can satisfy an applied load while the associated hydrogen-producing fuel processing assembly is not presently capable of doing so. One approach is to include one or more batteries or other suitable energy storage devices that may be used to satisfy the applied load until the fuel processing assembly can produce sufficient hydrogen gas for the fuel cell system to produce therefrom a sufficient power output to satisfy the applied load. However, the capacity of such a battery or other energy-storage device is limited, and it is therefore necessary for the hydrogen generation assembly to adjust its production rate of hydrogen gas as the demand for this hydrogen gas by the fuel cell stack increases. Similarly, when this demand decreases, it is desirable for the hydrogen generation assembly to reduce the rate at which hydrogen gas is produced. Some hydrogen generation assemblies may include a mechanism for consuming or otherwise utilizing excess hydrogen gas, but the overall efficiency of the assembly is generally reduced when excess hydrogen gas is generated.